Extract branch changes calculation logic into separate class

Summary

As a follow-up to !198736 (merged) (LFS mirroring timeout issue), we should extract the branch changes calculation logic from Projects::UpdateMirrorService into a dedicated class for better separation of concerns and maintainability.

Background

In !198736 (merged), we added logic to calculate which branches have changed during mirroring to optimize LFS object processing. Currently, this logic is embedded within the UpdateMirrorService, but it doesn't directly belong to the mirroring logic and could be extracted for better code organization.

Proposal

Create a new service class to handle branch changes calculation: ⚠️ suggested by Duo ⚠️

# app/services/projects/repository/branch_changes_calculator.rb
module Projects
  module Repository
    class BranchChangesCalculator
      def initialize(project)
        @project = project
      end

      def calculate_changed_revisions(local_branches)
        # Move the logic from UpdateMirrorService here
      end

      private

      def get_remote_branches
        # Extract from UpdateMirrorService
      end
    end
  end
end

Benefits

• Single Responsibility: Separates branch comparison logic from mirroring orchestration
• Testability: Easier to unit test branch calculation logic in isolation
• Reusability: Could be used by other services that need to detect branch changes
• Maintainability: Cleaner, more focused code in both classes

Implementation Details

The new class should handle:

• Comparing local vs remote branch states
• Generating appropriate revision formats (commit ranges vs full refs)
• Applying thresholds for fallback to --all

Related

• Original MR: !198736 (merged)
• Related issue: #415229 (closed)